It is well known that the feed gas for ammonia synthesis must be clean H2 and N2 during the production of an ammonia plant. But the content of H2 and N2 is from 91% to 97%, and that of CO and CO2 is from 1% to 8% in general feed gas, which also contains CH4 and Ar etc. The CO and CO2 components must be removed before the feed gas enters the return circuit of the ammonia synthesis, to bring the level of impurities in the feed gas to ppm levels, otherwise the catalyst for the ammonia synthesis will be poisoned and deactivated. Generally, the purifying methods for the feed gas are the copper washing method, the deep hypo-transforming methanation method, the methanolizing and methanating method, and the liquid-nitrogen washing method. The copper washing method is technically developed, and widely used domestically, but it consumes copper, acetic acid, ammonia and a lot of steam. The liquid waste contains the heavy metal Cu, which contaminates the environment. The liquid-nitrogen washing method mainly applies to ammonia plants that use coke-oven gas as the feed gas. The purity of the processed feed gas is high, but an air-separating device is needed with it, and the range of use is limited. Regarding the deep hypo-transforming methanation method, the limit of sulfur content in the feed gas is critical, and the amount of steam used is large. Because the place of origin of coal and its sulfur content varies, this method is not applicable to medium- and small-sized chemical fertilizer plants, which take coal as the raw material.
Regarding the refining process for the feed gas, the replacement of the deep hypo-transforming methanation method or copper washing method with the fine desulfurization-methanolization-methanation method, with the byproduct of methanol, was firstly presented in Chinese patent No. ZL90105545.X, “Refinery Process for Feed Gas for Ammonia Synthesis” by Xie Ding-zhong etc of China Hunan Anchun High Tech Co., Ltd in September 1990. This process was formally put into operation in 1992. It runs very well and has distinct advantages, with a pressure range of only 5 to 15 MPa. Furthermore, it gives high purity, requires little material and energy consumption, and has wide applicability. Later, Haldor Topsoe of Denmark also suggested the adoption of the methanation process in an ammonia plant in 1991. A plant was built in Egypt in 1992, and planned to go into production in 1993. But the pressure of this method is 22 MPa, the moisture content in methanol is up to 40% to 50%, and external concurrent heating is needed to maintain the reaction. In Chinese patent No. ZL93105920.8, “Equal pressure device for the joint production of methanol and ammonia”, it is presented that methanol synthesis and ammonia synthesis proceeded at the same high pressure. So the investment is large, the energy consumption is high, and the liquid nitrogen washing, copper washing or methanation methods are still needed for the refining of the feed gas.
The traditional methanation method above is used to make the purity of the feed gas meet the requirements of ammonia synthesis, and to avoid poisoning and deactivating the catalyst for ammonia synthesis. The content of CO and CO2 in the feed gas can be refined to from 5 to 15 ppm. During the refining of the feed gas, the content of CH4 will increase, but it won't take part in the ammonia synthesis reaction, and will accumulate gradually in the ammonia synthesis system and cause a pressure rise. When the pressure comes up to the safety control pressure, it is necessary to remove some gas mixture. When CH4 is emptied, the effective gases in the gas mixture, such as H2, N2 and NH3 etc, are lost at the same time. Therefore, the generation and entry of CH4 should be avoided and decreased as much as possible during ammonia synthesis.
China is a country with rich coal resources and relatively insufficient oil. How to transfer rough solid mineral energy into relatively clean liquid fuel is also a problem to be solved urgently. However, there is no effective and economical ammonia synthesis technology to transfer most CO and CO2 in the feed gas into high quality liquid fuels and multiple kinds of fine chemicals, and little into methane.